1. Field of the Invention
This invention relates to a thin-film crystal wafer having a pn junction and a method for fabricating the wafer.
2. Background Art
The heterojunction bipolar transistor (HBT) is a bipolar transistor whose emitter injection efficiency is enhanced by using an emitter layer material with a greater band gap than the base layer material in order to make the emitter-base junction a heterojunction. The HBT is a semiconductor device that is highly suitable for use in the microwave band and higher frequency regions. Owing to advantages such as ability to be driven by a single power source, high efficiency and low distortion, moreover, the HET is expected to be the semiconductor device of choice especially in next generation mobile telephone units.
Among the different types of HBT in actual use, the InGaP-system HBT is a particularly focus of attention owing to its superiority over the AlGaAs-system HBT, which has achieved the highest level of development to date, in features such as availability as high-purity crystal owing to good oxidation resistance, little reverse injection of holes owing to large valence electron band offset relative to GaAs, absence of the DX center (the deep-level electron trap typical of III-V Group compound semiconductors), low recombination speed at the interface, and advantages from the aspect of device processing due to large etching selection ratio.
The InGaP-system HBT is, for instance, produced using a thin-film crystal wafer given a structure wherein the pn junction constituting the emitter-base junction is formed as a heterojunction by using the metal-organic chemical vapor deposition (MOCVD) method to grow in order on a semi-insulating GaAs substrate an n+ GaAs layer, an n GaAs layer (collector layer), a p GaAs layer (base layer), an n InGaP layer (emitter layer), an n+ GaAs layer, and, topmost, an n+ InGaAs layer.
The basic operations of the HBT fabricated using the InGaP thin-film crystal wafer of this configuration are the same as those of the conventional bipolar transistor using silicon. In order to enable the HBT to provide adequate performance matched to specifications required in the microwave band and higher frequency regions, therefore, the HBT must be designed to have current amplification factor β and offset voltage Vbe values suitable for the specifications.
Since the current amplification factor characteristic and the offset voltage characteristic both depend on the energy band gap at the pn junction, the energy discontinuity value of the conduction band has to be changed in order to set these transistor characteristics to the desired values. In the case of an InGaP-system HBT, however, electrical characteristics suitable for the requirements cannot always be realized because the degree of freedom in designing the InxGa1−xP/GaAs system of the HBT is severely limited by the fact that lattice matching between the InxGa1−xP emitter layer and the GaAs base layer can be obtained only at a value of x=0.48.
Unexamined Japanese patent application JP-A-8-241896 teaches a method for overcoming this problem of the InGaP/GaAs-system HBT by selecting the x value of the In component of the InxGa1−xP emitter layer so that the InxGa1−xP lattice constant becomes slightly smaller or slightly larger than the lattice constant of the InGaAs base layer or the GaAs base layer.
When the x value of the In component of the emitter layer is significantly changed in accordance with this technique, however, compressive or tensile stress acting on the emitter layer and the base layer owing to the consequent large mismatch between the lattice constants of the two layers produces defects at the interface. The current amplification factor β decreases as a result. In addition, increased overall wafer warp caused by heavy strain produced throughout the wafer becomes a drawback during fine patterning. It is therefore difficult to achieve a substantial improvement in HBT design freedom with this prior art technology.